Method For Vaporizing Cryogenic Liquid Through Heat Exchange Using Calorigenic Fluid

ABSTRACT

A method of vaporizing cryogenic liquid, for example liquefied natural gas, by heat exchange with a calorigenic fluid, for example gaseous nitrogen is provided.

The present invention relates to a method of vaporizing cryogenicliquid, for example liquefied natural gas, by heat exchange with acalorigenic fluid, for example gaseous nitrogen.

In order to heat up and vaporize cryogenic liquids of the liquefiednatural gas (LNG) or equivalent type against a calorigenic fluid, inorder to recover the cold energy from the LNG, use has, in the past,been made of one of the following three options:

-   -   a technology that consists in coiling into the form of a pancake        coil a system comprising two tubes that are joined together by a        tie. The tubes are welded or expanded onto manifolds transverse        to the pancake coils;    -   brazed plate and fin exchangers;    -   wound tubular exchangers.

If there is a wish to recover the cold energy in order to liquefy airgases, it is absolutely essential to avoid accidental contamination ofthe nitrogen or the oxygen with a hydrocarbon gas, especially when thenatural gas is circulating through the exchanger at a pressure higherthan that of the air gas.

Tubular geometries are not very thermally efficient and often lead tocostly over engineering.

Moreover, existing methane terminals and existing air separation plantsdo not always have the equipment required to avoid sharp thermaltransients during shut downs and restarts, and this leads to thermalshock and therefore damage to the exchangers.

One subject of the invention is a method for heating up a first fluid byheat exchange with a second fluid in a plate and fin heat exchanger inwhich the first fluid is heated up in a first series of separatedpassages and the second fluid is cooled down in a second series ofseparated passages, characterized in that each passage of the firstseries is separated from the nearest passage of the second series by anauxiliary passage containing fins in which an inert gas circulates.

Optionally:

-   -   the first fluid consists of liquefied natural gas which is        vaporized or heated up in the first series of separated        passages;    -   the second fluid consists of gaseous nitrogen which is cooled        down or liquefies in the second series of separated passages;    -   the inert gas is at a pressure of at least 0.1 bar or even at        least 0.5 bar higher than those of the first fluid and of the        second fluid;    -   the inert gas is at a pressure of at least 0.1 bar, or even at        least 0.5 bar lower than those of the first fluid and of the        second fluid;    -   the inert gas is gaseous nitrogen;    -   the inert gas sent into at least certain auxiliary passages is        then sent to the atmosphere or to a flare;    -   at least one inlet and/or outlet box of one of the first and        second fluids is separated from the passages in which the other        of the first and second fluids circulates by means of a double        bar system, the bars possibly being separated by a dead zone;    -   the first fluid is heated up at a pressure of at least 60 bar        abs.

Another subject of the invention is a method of starting a plate and finheat exchanger in which, at full capacity, a first fluid is heated up byheat exchange with a second fluid in a plate and fin heat exchanger, thefirst fluid being heated up in a first series of separated passages andthe second fluid being cooled down in a second series of separatedpassages, characterized in that each passage of the first series isseparated from the nearest passage of the second series by an auxiliarypassage containing fins and in which during start-up an inert gas at atemperature lower than ambient temperature, possibly at cryogenictemperature, is sent into at least one auxiliary passage in order to getdown to temperature more swiftly.

The invention will be described in greater detail with reference to thefigures.

FIGS. 1 to 3 show a cross section, taken in the lengthwise direction ofthe exchanger, of each type of passage for an exchanger operating inaccordance with the invention. FIG. 1 depicts an auxiliary, inert gaspassage, FIG. 2 an LNG passage, and FIG. 3 a passage for nitrogen thatis to be heated.

FIGS. 4 to 6 depict another exchanger operating in accordance with theinvention. FIG. 4 shows a cross section through the parallel passages ofthe exchanger in the widthwise direction of the exchanger, FIG. 5 showsa low-pressure nitrogen passage sectioned in its lengthwise directionand FIG. 5 shows an LNG passage section in its lengthwise direction.According to the invention, a passage of the type of FIG. 1 will bepositioned between each passage of the type of FIG. 2 and each type ofFIG. 3. Thus, each passage of series of the type of FIG. 2 is separatedfrom each passage of the series of the type of FIG. 3 by a passage ofthe type of FIG. 1 to form a brazed plate and fin exchanger made ofaluminum or some other material. In order to simplify the drawing, thefins have not been illustrated.

FIG. 1 is the auxiliary, low-pressure inert gaseous nitrogen passage,the inlet 9 of which is bottom right and the outlet 11 of which is topleft.

FIG. 2 illustrates a passage for heating up liquefied natural gas (LNG)which enters the passage at bottom left 1 and emerges top right 3. Adouble bar isolates the top and the bottom of the LNG passage from theinert nitrogen passage.

FIG. 3 shows a passage for the cooling of high-pressure gaseous nitrogenwhich enters the top of the passage through the inlet 7 and emerges atthe bottom through the outlet 5. The high-pressure gaseous nitrogenpassage is not as wide as the low-pressure nitrogen passages of FIG. 1or as the liquefied natural gas passages of FIG. 2.

To avoid the nitrogen becoming contaminated with the liquefied naturalgas, an auxiliary passage is interposed between each pair of nitrogenand LNG passages. The exchange of heat between the nitrogen and LNGpassages will be via the fins of the auxiliary passage, by conduction.Obviously, the corrugation chosen for the auxiliary passage will have anoptimal height/thickness ratio.

In the case illustrated, the auxiliary passages will be swept withlow-pressure gaseous nitrogen (at a pressure lower than that of the LNGof FIG. 2 and than that of the nitrogen of FIG. 3) and collected fordischarging to the atmosphere or possibly to a flare.

The boxes which cover the stack and may therefore be sources ofcontamination, will therefore be isolated from the other fluid usingdead zones Z.

Gas from the dead zones Z will be collected, and these zones maypossibly be swept with low-pressure nitrogen.

The above dead zones may be isolated from the LNG and nitrogen circuitsby means of a double bar 2 system in order to improve sealing. Gasesfrom the clearance space between the double bars 2 may itself becollected in order to improve intrinsic safety. This is explained ingreater detail in respect of the method of FIGS. 5 and 6 but appliesequally to the method of FIGS. 1 to 3.

The passages of FIG. 1 are used during start-ups to bring the exchangerdown to temperature gradually and in a controlled manner using a flow oflow-pressure nitrogen drawn from an auxiliary volume.

According to another aspect of the invention which is illustrated inFIG. 4, each passage for nitrogen that is to be heated up (N₂ LP) isisolated from the passages for LNG to be vaporized by a passagecontaining a high-pressure inert process gas (N₂ HP), in this casenitrogen at a higher pressure than the nitrogen that is to be heated up(35 bar) and than the liquefied natural gas that is to be vaporized (15bar).

As can be seen in FIGS. 5 and 6, the bars which separate a circuit fornitrogen that is to be heated up from an LNG circuit are duplicated sothat the space between them forms a dead zone Z open to the atmospherevia a vent V, so that any leak of liquefied natural gas can escape viathis route. The passages in FIGS. 5 and 6 are separated by ahigh-pressure inert gas passage.

1-10. (canceled)
 11. A method for heating up a first fluid by heatexchange with a second fluid in a plate and fin heat exchanger in whichthe first fluid is heated up in a first series of separated passages andthe second fluid is cooled down in a second series of separatedpassages, comprising: separating each passage of the first series fromthe nearest passage of the second series by an auxiliary passagecontaining fins in which an inert gas circulates.
 12. The method ofclaim 11, wherein the first fluid consists of liquefied natural gaswhich is heated in the first series of separated passages.
 13. Themethod of claim 11, wherein the first fluid consists of liquefiednatural gas which is vaporized in the first series of separatedpassages.
 14. The method of claim 11, wherein the second fluid consistsof gaseous nitrogen which cools in the second series of separatedpassages.
 15. The method of claim 11, wherein the second fluid consistsof gaseous nitrogen which liquefies in the second series of separatedpassages.
 16. The method of claim 11, wherein the inert gas is at apressure at least 0.1 bar higher than those of the first fluid and ofthe second fluid.
 17. The method of claim 11, wherein the inert gas isat a pressure at least 0.5 bar higher than those of the first fluid andof the second fluid.
 18. The method of claim 11, wherein the inert gasis at a pressure at least 0.1 bar lower than those of the first fluidand of the second fluid.
 19. The method of claim 11, wherein the inertgas is at a pressure at least 0.5 bar lower than those of the firstfluid and of the second fluid.
 20. The method of claim 11, wherein theinert gas is gaseous nitrogen.
 21. The method of claim 11, wherein theinert gas sent into at least certain auxiliary passages is then sent tothe atmosphere.
 22. The method of claim 11, wherein the inert gas sentinto at least certain auxiliary passages is then sent to a flare. 23.The method of claim 11, wherein at least one inlet and/or outlet box ofone of the first and second fluids is separated from the passages inwhich the other of the first and second fluids circulates by means of adouble bar system.
 24. The method of claim 23, wherein said double barsystem comprises bars separated by a dead zone.
 25. The method of claim11, wherein the first fluid is heated up at a pressure of at least 60bar abs.
 26. A method of starting a plate and fin heat exchanger inwhich, at full capacity, a first fluid is heated up by heat exchangewith a second fluid in a plate and fin heat exchanger, the first fluidbeing heated up in a first series of separated passages and the secondfluid being cooled down in a second series of separated passages,comprising separating each passage of the first series from the nearestpassage of the second series by an auxiliary passage containing fins andin which during start-up an inert gas at a temperature lower thanambient temperature, possibly at cryogenic temperature, is sent into atleast one auxiliary passage in order to get down to temperature moreswiftly.
 27. The method of claim 26, wherein during start-up said inertgas has a cryogenic temperature.